1. Technological Field
The present disclosure relates generally to the field of delivery of digital data (e.g., text, video, and/or audio) over optical networks or any network utilizing optically-combined simultaneous transmissions from lasers of identical or nearly-identical wavelengths. Specifically, various methods and apparatus of the present disclosure are directed to removing beat interference from the splitting and/or combining of signals.
2. Description of Related Technology
Service providers, or more generally multiple system operators (MSOs), continually strive to increase the data capacity of their networks to increase revenue. Many MSOs have migrated to optical technologies by replacing the coax portion of existing Hybrid Fiber Coax (HFC) networks with a single-fiber Passive Optical Network (PON). Exemplary networks utilize different optical wavelengths in the downstream (e.g., to the consumer premises) and return-path directions (e.g., from the consumer premises). Typically, the optical networks use 1550 nm downstream, and 1310 nm or 1590/1610 nm upstream wavelengths. The 1590/1610 nm return path allows the fiber infrastructure to support both RFoG (Radio Frequency over Glass) and PONs simultaneously.
In conventional RFoG optical communication networks, so-called Optical Network Units (ONUs) are typically deployed at each of multiple customer premises locations. In a conventional RFoG network, a single strand of optical fiber is typically shared among multiple downstream ONUs (typically 32, but other numbers are possible). In the downstream direction, a light splitting resource divides downstream light power to the ONUs such that a portion of the downstream light power is transmitted to each ONU. Each of the ONUs receives light containing identical information in the downstream direction (from the cable network company to a corresponding subscriber customer). Typically, a signal analyzer analyzes the received signal to determine which data is directed to the corresponding subscriber. In certain instances, a portion of content encoded on a respective downstream optical signal can include data available for consumption by multiple subscribers.
For upstream transmissions (customer to company), each of the ONUs can include a respective laser transmitter that is manufactured to identical specifications. As such, the transmitters transmit on the same or nearly identical wavelength of light in the upstream direction to the company. When two downstream located laser transmitters transmit at the same time in an upstream direction, the optical receiver at the company side facility (such as at a cable modem termination system) receives optical power from both laser transmitters. An optical detection device in the upstream device converts the optical signal into a respective electrical output that is proportional to the instantaneous sum of the combined optical powers contributed by the two lasers. Since the wavelengths of these low-cost lasers (e.g., the upstream optical transmitters in the ONUs) are not precisely controlled, in most cases they are separated in the optical spectrum by a spacing that is sufficient to consider only the sum of the optical powers at the receiver output because there is a tolerance in the upstream wavelengths (per the RFoG specification) where 1310 nm can vary by +/−50 nm and 1610 nm can vary by +/−10 nm. So largely variability will not exceed those tolerances hence the conclusion that the wavelengths would be the sum of optical powers. If the RF (Radio Frequency) signals feeding the two ONUs are on different RF channels, both channels will appear in the electrical output generated by the upstream located receiver.
RFoG delivers the same services as HFC network, with the added benefit of improved noise performance and increased usable RF spectrum in both the downstream and return-path directions. Since both RFoG and HFC systems can concurrently operate out of the same headend/hub, service providers can leverage existing capital infrastructure. More generally, cable operators can continue to use existing provisioning and billing systems, Cable Modem Termination System (CMTS) platforms, headend equipment, set-top boxes, conditional access technology and cable modems.
However, RFoG does suffer from certain unusual interference effects. Specifically, in converting the RF signal to optical signaling, the RF channel separation is lost. Instead, the converted RFoG optical signaling is performed on substantially the same wavelength (accounting for imprecise manufacturing tolerances of low-cost lasers). When upstream signals from multiple lasers are combined in a passive splitting device (as is typically done in modern RFoG networks), so-called “beat frequency” effects may occur because the wavelengths are very close in frequency. Such interference can become severe to the point where no communication is possible until the event (collision) passes. More generally, “Optical Beat Interference” (OBI), is an issue with any point to multi-point optical system where the laser transmitters are nearly identical in wavelength, and where multiple transmitters may transmit simultaneously. OBI is well-documented in real-world RFoG systems.
Existing networks are unable to prevent OBI, and instead rely on the statistical rarity of collisions and collision resolution schemes. For example, when multiple lasers simultaneously (or nearly simultaneously) transmit, the resulting OBI will interfere with proper reception. Consequently, both transmitters back-off for a randomized time interval (similar to Carrier Sense Multiple Access with Collision Detection (CSMA/CD) schemes used in e.g., Ethernet applications) and retransmit thereafter. However, such solutions severely limit throughput in many common traffic scenarios and are thus undesirable. Additionally, retransmission cannot be used with certain types of protocols (e.g., User Datagram Protocol (UDP)).
Accordingly, improved apparatus and methods are needed to address the foregoing, including reducing or removing the effects of OBI in optical networks. More generally, various aspects of the present disclosure are broadly applicable to any interference effects which are the result of passive mixing (e.g., beat frequencies, parasitic noise, etc.)